Secular variations in chemical composition, temperature and isotopic ratio in Zenikawa Hot Spring can be explained by changes in mixing ratios of three components, i. e. a Na-Cl type thermal water which is same as the thermal water flowing from Iwanoyu before later 1950's, a shallow ground water similar to cold water from the wells for prevention of land sliding, and a steam seperated from the Na-Cl type thermal water through boiling underground. The variations were caused by two successive changes in the mixing ratios. In the first change, the fraction of Na-Cl type thermal water was decreasing and inversely the fractions of shallow ground water and steam were increasing. This change is due to boiling of the Na-Cl type thermal water and mixing of the shallow ground water, which were caused by pressure decreasing of the thermal water underground. After 1975, the ratio of the thermal water and the steam became constant. The second change happened following a drastic change of discharge rate during 1979 to 1982. In this change, the fraction of Na-Cl type thermal water turned increasing. This suggests that the pressure decreasing of shallow ground water was more than that of the Na-Cl type thermal water.